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TMS320F28335: Issue with Frequency Output in custom PCB Based on TMS320F28335PTPS Using TI XDS 110 Debug Probe.

Atul saurabh
Prodigy 10 points
Part Number: TMS320F28335

Tool/software:

I have developed a Custom PCB based on the F28335PTPS microcontroller and am using the TI XDS 110 Debug Probe for JTAG debugging. While generating waveforms at a specific frequency, I’ve observed that the output frequency on the hardware is approximately one-third of the expected value. However, when simulating the same setup in Simulink, the output frequency is correct.

Key details:

  • Microcontroller: TMS320F28335PTPS
  • Debug Probe: TI XDS 110 (JTAG)
  • Crystal: 30 MHz

Upon further investigation, I noticed that the crystal frequency, as observed on the DSO, fluctuates rapidly between Hz and MHz instead of maintaining a stable value. This irregularity leads me to suspect there might be an issue related to the clock configuration or the crystal itself.

I’ve attached the schematics for your reference. I would greatly appreciate it if you could provide insights into potential causes and suggestions to resolve this issue.

Looking forward to your expert guidance.

  • 0
    TI__Expert 4705 points

    Hello,

    Can you share a screenshot of the clock irregularities and the code you are using to initialize the PLL?

    Best,

    Matt

  • 0 in reply to Matt Kukucka
    Prodigy 10 points

    ’m trying to generate a 5 kHz signal using GPIO and EPWM , and while the Scope shows the correct 5 kHz frequency, the hardware output shows around one-third of that frequency. There’s no PLL involved in the setup. i have attached scope output and code images.

    i am generating square wave of of freq 5KHZ for testing GPIO pin of PCB

    scope freq is 5KHZ

  • 0 in reply to Atul saurabh
    TI__Mastermind 23570 points

    Hi Atul,

    Atul saurabh said:
    and while the Scope shows the correct 5 kHz frequency, the hardware output shows around one-third of that frequency

    Can you please help clarify what you are scoping and what you mean by 'hardware output'? Are these two different sources? 

    Atul saurabh said:
    This irregularity leads me to suspect there might be an issue related to the clock configuration or the crystal itself

    What is your clock configuration setup?

    Atul saurabh said:
    m trying to generate a 5 kHz signal using GPIO and EPWM

    Do both of these methods give you one-third of that frequency?

    Best Regards,

    Marlyn

  • 0 in reply to Marlyn Rosales Castaneda20
    Prodigy 10 points

    I’m generating a 5 kHz signal, and it shows the correct frequency on the Scope. However, when I dump the code into the MCU (F28335), the frequency becomes one-third of the expected value. hardware output means output from MCU after dumping code and from same sources. and both of these methods gives me one third of that frequency.

  • 0 in reply to Atul saurabh
    TI__Expert 4705 points

    Hi,

    What are your SysCtrl settings i.e. how do you initialize the peripheral clocks? The frequency being 1/3 the expected value makes me suspect that the code is assuming a 10 MHz clock, not 30 MHz.

    Best,

    Matt

  • 0 in reply to Matt Kukucka
    Prodigy 10 points

    Actually, when I dump the code onto another hardware, like the F28379D LaunchPad, the output is correct as expected. So, the SysCtrl settings and everything related to code else are correct.

  • 0 in reply to Atul saurabh
    TI__Expert 4705 points

    Hi,

    The system control clocking is quite different between F2837xD and F2833x devices, although it's interesting to know that it worked on the F2837xD LaunchPad.

    Can you please still share what your SysCtrl settings are for the F2833x code, specifically what you're configuring in the PLLCR and PLLSTS registers?

    Best,

    Matt